Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to a heart to cause a heart, which would normally or otherwise beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functionalities of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation therapy when it is required.
As is well known, a cardiac cycle on an electrocardiogram (ECG) extends from one heart beat (QRS complex) to the next. During each cardiac cycle, a T wave occurs. The T wave is a low-frequency wave that follows the ST-segment and represents repolarization of the ventricular myocardium. Alternate occurring T waves (odd and even) are referred to as T wave alternans.
T wave alternan patterns are known to be a precursor for sudden cardiac death. In the past, detection of T wave alternan patterns has been performed using surface ECGs. Implementation of such detection has included the measurement, on a beat-to-beat basis, of the micro-volt level changes in the T wave amplitude from the surface ECG. Then, the long record of time series of T wave amplitude change is transformed into the frequency domain by fourier series transformation (FFT). A prominent peak in the FFT at 0.5 Hz would verify the existence of a T wave alternan pattern.
Unfortunately, the above detection method requires the use of medical equipment that must be operated by medical personnel in a medical facility such as a physician's office. The detection requires long term recording of surface ECGs and off-line analysis with robust computation equipment. As a result, T wave alternan pattern monitoring has been inconvenient and cumbersome. As a result, it is difficult to provide continuous and regular T wave alternan pattern monitoring.
Many patients who would benefit from T wave alternan pattern monitoring have an implanted cardiac device such as an implantable defibrillator or a combined defibrillator pacemaker. It would thus be highly desirable if such an implanted device could detect for T wave alternan patterns. However, the prior art detection method does not lend itself for such application due to, for example, the required long term monitoring, surface ECG, and robust computational requirements for fourier series transformation.
In order for an implanted cardiac device to provide T wave alternan pattern monitoring, there is a need for a new and different approach. The present invention addresses that need.